Capping layer preventing deleterious effects of As--P exchange

ABSTRACT

A III-V semiconductor heterojunction in which a capping layer (14) is formed between the two layers (10, 16) of the heterojunction to prevent any deleterious effects due to As--P exchange. When InAlAs is grown on InP, the capping layer is AlP. When GaAs is grown on GaInP, the capping layer is GaP.

FIELD OF THE INVENTION

The invention relates generally to compound semiconductor devices. Inparticular, the invention relates to stabilizing an interface of acompound-semiconductor heterostructure including GaAs or InP.

BACKGROUND ART

Many advanced semiconductor electronic and opto-electronic devices arebeing developed based on the InP family of semiconductors. An example isa fast transistor using InP channels that take advantage of the highsaturated electron velocity in InP. In this case, In₀.52 Al₀.48 As isgrown over an InP substrate or InP epi-layer. The band bending at theInAlAs/InP interface leads to two-dimensional confinement of electronson the InP side of the interface and of holes on the InAlAs side.Another example is a thin quantum well of InAs grown on an InP barrierand then covered with another InP barrier. The bandgap in the quantumwell corresponds to about 1.3 μm of optical energy, a region of greatinterest for optical fiber communications. In both examples, theinterfacial characteristics are crucial because the interesting effectsare occurring within a few nanometers of the nominal interface.

Such structures can be grown by a variety of techniques, molecular beamepitaxy (MBE), organo-metallic chemical vapor deposition (OMCVD), ororgano-metallic molecular beam epitaxy (OMMBE), which is a combinationof the first two and is sometimes referred to as chemical beam epitaxy(CBE). The conventional OMMBE method will be described with reference toan InAs/InP interface. An InP epi-layer is deposited by exposing an InPsubstrate to a medium pressure of trimethyl-indium (TMI) by theevaporation of TMI (a solid at room temperature) and to a mediumpressure of molecular phosphorus (P₂) obtained by cracking phosphine ina cracker cell. The TMI cracks on the hot substrate surface into Inwhich then combines with the arriving P₂ to form the desired InP. At theinterface, the beam of phosphorus is interrupted and is replaced by abeam of molecular arsenic (As₂) from another cracker cell crackingarsine to similarly form the desired InAs. If the second layer is to becomposed of InAlAs, tri-isobutyl-aluminum (TIBAl) is supplied by theevaporation of TIBAl (a liquid at room temperature) in a fixedproportion of the TMI. Because the absence of the group-V flux isbelieved to degrade the surface quality, it is common practice tointerrupt the TMI for about two seconds during the switching from P₂ toAs₂ and before supplying the TIBAl in order to assure that the group-Velement is not a combination of P and As.

Our experience, however, has shown that the InAs/InP interface is lessabrupt than would be expected from the above sequence. We attribute themajor part of the poor interface to an exchange between the alreadygrown phosphide with the after supplied As. Indeed, we have grown alayer of InAs that was between 5 and 10 monolayers thick by simplyexposing the InP to As₂ for eight seconds in the absence of TMI. Thethickness though was spatially varying. Such interfacial roughnessundesirably scatters electrons, thus limiting device performance. It ispossible that such As--P exchange continues after the fabrication of theinterface, thereby bringing into question the long-term reliability of adevice utilizing such an interface. Obviously, the poor interfaceabruptness and questionable reliability are not satisfactory.

A similar problem has arisen in GaAs/GaInP heterostructures, which areimportant for modulation-doped field-effect transistors, heterojunctionbipolar transistors, lasers, and solar cells. The alloy Ga₀.515 In₀.485P is lattice matched to GaAs. A two-dimensional electron gas forms onthe GaAs side of the heterojunction, as disclosed by Razeghi et al. in"Extremely high electron mobility in a GaAs--Ga_(x) In_(1-x) Pheterostructure grown by metalorganic chemical vapor deposition",Applied Physics Letters, volume 55, 1989, pp. 457-459. In a superlatticeconfiguration, GaAs forms the quantum well. Such GaAs/GaInPheterostructures can be grown by OMMBE, by gas source MBE (GSMBE), or byOMCVD. The latter method is often preferred because of its simplicity,economy, and high throughput. However, changing over the group-V elementis known to present a problem. The P cannot be shut off in a cessationof growth during which the chamber is flushed because its high vaporpressure would cause it to evaporate from the already deposited GaInP.Bhat et al. have described the deleterious reaction of arsine andphosphine with underlying III-V materials, especially GaAs and InP, in"A novel technique for the preservation of gratings in InP and InGaAsPand for the simultaneous preservation of InP, InGaAs, and InGaAsP inOMCVD," Journal of Crystal Growth, volume 107, 1991, pp. 871-877. Thatarticle disclosed that neither InP nor InGaAs is stable in arsine orphosphine respectively. Thus, when GaAs is grown on GaInP, theconventional technique of immediately substituting arsine for phosphineis likely to produce an intermediate layer of GaInAsP of gradedcomposition and having an uneven surface. Such a layer can have a lowerbandgap than GaAs and so will capture free charge carriers, principallyheavy holes. Therefore, recombination occurs close to the interfacewhere the material's wavelength is longer than desired. Furthermore, theefficiency is likely to be reduced because of nonradiative centersarising from the interfacial roughness. Also, the graded compositionsmears the interface, a critical problem when the GaAs layer is thin soas to serve as a quantum-well layer. That reference also suggested thatAs₄ rather than As₂ should be used to preserve InP during its heat upbefore growth.

SUMMARY OF THE INVENTION

The invention can be summarized as a method of stabilizing the interfacebetween a III-V semiconductor layer containing P and a III-Vsemiconductor layer containing As grown over it. Instead of simplyswitching the P to As, an atomically thin capping layer is grown whichcontains P, for which any reaction with the growth reactant for the Asis non-deleterious. Preferably, the capping layer should have a widebandgap and have a higher bond strength than the overgrown layer. ForInAlAs grown on InP, an AlP capping layer is interposed. For GaAs grownon GaInP, a GaP capping layer is interposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a first embodiment of the inventionincluding an aluminum stabilized interface in which InAlAs is grown onInP.

FIG. 2 is a sequencing chart illustrating the timing in the supply ofprecursors according to the invention in the fabrication of thestructure of FIG. 1.

FIG. 3 is a cross-section of a second embodiment of the inventionincluding a quantum-well structure having a capping layer between GaAsgrown on GaInP.

FIG. 4 is a sequencing chart illustrating the timing in the supply ofprecursors according to the invention in the fabrication of thestructure of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment of the invention is illustrated incross-section in FIG. 1. An InP epilayer 10 is epitaxially deposited onan InP substrate 12. A very thin AlP capping layer 14 is epitaxiallydeposited on the InP epilayer 10. An InAlAs layer 16 is epitaxiallydeposited on the AlP capping layer 14. The AlP is more refractory thanthe InP and then any As--P exchange from the InAlAs, during growth oruse, occurs only in a non-deleterious way.

The capping layer 14 should possess several characteristics. It need notbe lattice matched to the underlying layer 10, but it should be ofsufficient thinness that it is pseudomorphic to that layer 10. Thismeans that when the capping layer is epitaxially formed on itssubstrate, its lattice spacing within the layer plane matches the growthsubstrate. Beyond a critical pseudomorphic thickness, dislocationdefects form which allow any further growth to assume its natural orbulk lattice spacing. A pseudomorphically thin capping layer 14 allowsthe epitaxy of relatively thick layers 10 and 16 of lattice matchingcomposition and separated by the non-lattice-matched capping layer 14.The capping layer should have a thickness of at least a binary monolayerto provide the chemical isolation of the underlying layer, but thicknessof more than ten monolayers may cause undesired degrading of theheterojunction interface. The capping layer 14 should be composed of amaterial having a larger bandgap than the associated active layer 16 toprevent charge capture. In order to stabilize the interface, the atomicbonding strength of the compound constituents of the capping layer 14should be larger than those formed in the underlying layer. For an InPsubstrate, the P bonds more strongly with the Al anion than with the Inanion. A high heat of formation indicates a large bonding strength. Anatomically thin layer of AlP satisfies all these criteria when overgrown with InAlAs.

EXAMPLE 1

An example of the invention was grown using the timing chart given inFIG. 2. The structure of FIG. 1 was grown by OMMBE using a VacuumGenerators V80H gas source MBE. Trimethyl-indium (TMI) andtri-isobutyl-aluminum (TIBAl) were supplied from gas sources as thegroup-V precursors. Cracked arsine and phosphine were supplied from gassources as the group-III precursors in the form of As₂ and P₂. Allsources were selectively masked to change the composition of thedeposited material. The substrate temperature was preferably heldbetween 480 and 520 C. during growth.

The InP substrate 10 was vicinally oriented at 2° off the (100) surfacetoward (011) and was doped n⁺ with S. Alternatively, it could be dopedwith Fe to be semi-insulating. The InP buffer layer 12 was grown to athickness of about 100 nm during a first deposition step 20 during whichthe In and P sources were turned on. Then, the AlP stabilization layer14 was deposited in a second deposition step 22 in which the In anionsource was turned off, the Al anion source was turned on, and the Pcation source continued on. The second step 22 continued for about 10seconds so that the AlP formed to a thickness of about a binarymonolayer. In a growth halt 24, the In, Al, and P sources were allturned off, but the As source was turned on. During the growth halt 24,the growth chamber was purged of P and filled with As, but no mixedgrowth occurred because of the absence of In and Al. A 2-sec growth halt24 seemed satisfactory although longer halts did not degrade thestructure. The InAlAs layer 16 was grown in a fourth deposition stepduring which both the In and Al anion sources were turned on in adesired ratio and the As source remained on. The In_(x) Al_(1-x) Ascomposition was chosen to be x=0.52 so that the InAlAs was nearlylattice matched to InP. The monolayer thickness of the AlP layer 14 isan extreme of a pseudomorphic thickness for which the AlP, althoughhaving a bulk lattice constant different from InP, is epitaxiallydeposited in a sufficiently thin layer that its lattice spacing withinthe layer plane matches the InP on which it is grown. Beyond a criticalpseudomorphic thickness, dislocation defects form which allow theoverlayer to assume its bulk lattice spacings. The capping layer 14 canhave a thickness of more than a monolayer, but thicknesses of more thanten binary monolayers may cause an undesired degrading of theheterojunction interface.

Photoluminescence data were obtained at 4K from both the inventivestructure and a conventional structure without the AlP stabilizationlayer 14, that is, fabricated without deposition step 22. The AlP layer14 increased energy of the emission peak by a few tens of meV over astructure grown without it. Increasing the growth halt after the AlPformation from 2 s to 8 s changed the results insignificantly.Importantly, an increase in the growth halt in the conventionalstructure substantially degraded the photoemission peak, significantlydecreasing the emission energy, and indicating a degradation of theinterface, which appears to be due to As--P exchange.

The invention may also be applied to the OMCVD growth of GaAs/GaInPheterostructures. In a second preferred embodiment illustrated in crosssection in FIG. 3, a GaAs quantum-well structure includes a lowerbarrier layer 30 of GaInP lattice matched to a GaAs substrate 32. Anextremely thin capping layer 34 of GaP is interposed between the GaInPbarrier layer 30 and a thin quantum-well layer 34 of GaAs. Thequantum-well structure is completed with a top GaInP barrier layer 38. AGaP capping layer is not required when the GaInP of the top barrierlayer 38 is grown over the GaAs quantum-well layer 36 although it couldbe readily added.

The growth sequence for the second embodiment is illustrated in FIG. 4.A first deposition step 40 supplies TMG, TMI, and phosphine into theOMCVD reaction chamber to form the GaInP barrier layer. In a growth halt42, both the TMG and TMI are interrupted. A second deposition step 44forms the few monolayers of the GaP capping layer by again supplying TMGbut not TMI. During these steps 40, 42, and 44, phosphine is suppliedwithout interruption. A third deposition step 46 forms the GaAsquantum-well layer by stopping the phosphine and supplying arsine. Anyreaction at the interface between the GaP and arsine would produceGaAsP, which has a higher bandgap than the GaAs, thus not affecting thequantum confinement. A growth halt is not required while substitutingarsine for phosphine between steps 44 and 46.

EXAMPLE 2

A superlattice structure was fabricated with many periods of a GaInPbarrier layer and a GaAs quantum-well layer. The interface transitioningfrom GaInP to GaAs, however, followed the quantum-well structure of FIG.3. The superlattice was grown by OMCVD at low pressure following theprocedures of the Bhat et al. article. The GaAs substrate had a (001)orientation. The barrier layers of Ga_(x) In_(1-x) P had an alloyingfraction of 0.515. The capping layer of GaP is estimated to have had athickness of about 0.8 nm, that is, about three monolayers. Thequantum-well layers had a thickness of 13 nm.

The structure of the invention was then tested for photoluminescence. Itdisplayed a single peak at an energy close to that predicted for thatsized quantum well. In comparison, when the structure was grown withoutthe GaP capping layer so that a graded interfacial layer of InGaAsP wasformed, photoluminescence exhibited peaks of lower energy than expected.The energy spacings in conventional structures between heavy-hole andlight-hole radiative transitions varied, indicating degraded interfaces.Furthermore, when the inventive and comparative samples were measuredfor X-ray rocking curves, the satellite peaks were extremely sensitiveupon the switching sequence.

The invention thus provides for an improved heterojunction between InPand InAlAs or GaAs and GaInP with only minor modification of the growthsequencing. The superior heterojunction provides significantly improvedquantum effects.

What is claimed is:
 1. A method for forming a III-V semiconductorheterostructure comprisinga first step of depositing upon a substrate afirst III-V semiconductor composition including a first group IIIelement and phosphorous to thereby form a first layer, a second step ofdepositing upon said first layer a thin capping layer of sufficientthinness to be pseudomorphic to said first layer and of sufficientthickness to provide chemical isolation of said first layer, saidcapping layer comprising a second III-V semiconductor compositionincluding a second group III element and phosphorous; and a third stepof depositing upon said capping layer a third III-V semiconductorcomposition comprising said second group III element and arsenic to forma second layer; whereby said capping layer interposed between said firstlayer and said second layer is a transition layer between the phosphidefirst layer and the arsenide second layer.
 2. A method as recited inclaim 1, wherein said first composition comprises gallium indiumphosphide, said second composition comprises gallium phosphide, and saidthird composition comprises gallium arsenide, wherein said first groupIII element comprises indium and said second group III element comprisesgallium.
 3. A method as recited in claim 2, wherein said firstdepositing step includes exposing said substrate to a gallium precursor,an indium precursor, and a phosphorus precursor, said second depositingstep includes exposing said first layer to said gallium precursor andsaid phosphorus precursor, and said third step includes exposing saidcapping layer to said gallium precursor and an arsenic precursor, andfurther comprising a growth halt step between said first and seconddeposition steps comprising exposing said first layer to said phosphorusprecursor but not to said gallium precursor nor to said indiumprecursor.
 4. A method as recited in claim 1 wherein phosphorous isatomically bonded more strongly to said second group III element than tosaid first group III element.
 5. A method of growing an InP/InAlAsheterostructure, comprising the steps of:a first step of depositing upona substrate a first III-V semiconductor composition comprising indiumphosphide to thereby form a first layer; a second step of depositingupon said first layer a second III-V semiconductor compositioncomprising aluminum phosphide to thereby form a second layer; and athird step of depositing upon said second layer a third III-Vsemiconductor composition comprising indium aluminum arsenide.
 6. Amethod as recited in claim 5, wherein said first depositing stepincludes exposing said substrate to an indium precursor and a phosphorusprecursor, said second depositing step includes exposing said firstlayer to an aluminum precursor and said phosphorus precursor, and saidthird step includes exposing said second layer to said indium precursor,said aluminum precursor, and an arsenic precursor, and furthercomprising a growth halt step between said second and third depositionsteps comprising exposing said first layer to said arsenic precursor butnot to said indium precursor nor to said aluminum precursor nor to saidphosphorus precursor.
 7. A method as recited in claim 5, wherein saidsecond step deposits said aluminum phosphide substantially directly uponsaid first layer and wherein said third step deposits said indiumaluminum arsenide substantially directly upon said second layer.